A series of experiments has been planned utilizing gnotobiotic rats to determine the relative cariogenicity of the different serotypes of S. mutans and certain S. mutans mutants of special interest. In the first experiment two germfree isolators were used, each containing 12 Osborne-mendel rats. The rats in one isolator were orally inoculated with a serotype d strain isolated from the high caries community of Don Matias (Colombia, S.A.). Rats in the second isolator were inoculated with a serotype c strain from the low caries community of Heliconia. After 8 weeks the animals were sacrificed. Saliva and blood was collected for analysis and the heads processed for caries scoring.